Electrostatic discharge (ESD) is a continuing problem in the design and manufacture of semiconductor devices. Integrated circuits (ICs) can be damaged by ESD events, in which large currents flow through the device. These ESD events can stem from a variety of sources. In one such ESD event, a packaged IC acquires a charge when it is held by a human whose body is electrostatically charged. An ESD event can occur when the IC is inserted into a socket, and one or more of the pins of the IC package touch the grounded contacts of the socket. This type of event is known as a human body model (HBM) ESD event. Another ESD event, which can be caused by metallic objects, is known as a machine model (MM) ESD event. An MM ESD event can be characterized by a greater capacitance and lower internal resistance than the HBM ESD event. A third ESD event is the charged device model (CDM). The CDM ESD event involves situations where an IC becomes charged and discharges to ground.
ESD events typically involve discharge of current between one or more pins or pads exposed to the outside of an integrated circuit chip. The direction of current flow from an ESD event results from positive or negative ESD strikes, which are determined from the polarity of voltage on the pad relative to ground or a supply voltage terminal. In either type of ESD event, positive or negative, current may flow through vulnerable circuitry in the IC that may not be designed to carry such currents. The vulnerability of IC chips to ESD strikes has created an important need for ESD protection circuits.
As a result of the need to protect IC chips from ESD strikes, ESD protection circuits are often added to the integral design of IC chips, such as RF power amplifiers. Many conventional ESD protection schemes for ICs employ peripheral dedicated circuits to carry the ESD currents from the pin or pad of the device to ground by providing a low impedance path. Thus, an output ESD cell requires low impedance for proper ESD protection. In this way, the ESD currents flow through the protection circuitry, rather than through the more susceptible circuits in the chip. However, ESD protection of RF power amplifiers has been historically difficult due to a competing requirement of low output capacitance for maximum RF power transfer.